1. Field of the Invention
The present invention relates to an ignition timing control system for internal combustion engines which is so designed that the occurrence of knocking in an engine is detected and the ignition timing is controlled in accordance with the result of the detection.
The ignition timing of the engine must be determined in accordance with the condition of the engine so as to ensure optimum operation of the engine.
2. Description of the Prior Art
The ignition timing control systems heretofore known have been generally so designed that the speed and intake vacuum of an engine are respectively detected by centrifugal and vacuum spark advance mechanisms so as to represent the condition of the engine and thereby to determine the ignition timing.
Generally, it has been considered that the best way from the standpoint of engine efficiency and fuel consumption is to effect the ignition at around the so-called point of minimum spark advance for best torque or MBT point and it has been necessary to vary the ignition timing to the MBT point in accordance with the engine conditions.
However, if the ignition timing is advanced under certain engine conditions, the engine will be caused to knock and thus the engine will be prevented from operating stably. Generally, the relation between MBT and the ignition timing at which knock occurs is such that the knocking limit is reached before MBT under low speed and low load operation of the engine. Also, the knocking limit tends to be affected by the atmospheric conditions such as the temperature, humidity, etc., and the known ignition timing control systems have been preliminarily programmed so that the ignition timing is maintained considerably later than MBT in accordance with the engine speed and intake vacuum parameters so as to prevent the occurrence of knock under all the engine operating conditions. As a result, the power output and fuel consumption of the engine are held lower than the engine performance.
To overcome these deficiencies and operate an engine under the best condition, a system has been proposed which is so designed that usually the ignition is effected in accordance with the ignition timing corresponding to the engine condition, and upon detection of knocking the ignition timing is controlled in accordance with the result of the detection so as to maintain a trace knocking condition and thereby to improve the power output and the fuel consumption.
FIG. 1 of the accompanying drawings is a diagram showing the control range of a known ignition timing control system of the knocking detecting type.
Referring to FIG. 1, in order to operate the engine under non-knocking or trace knocking conditions so as to improve the power output and fuel consumption of the engine, the control range must extend from the basic ignition timing (at A in the Figure) which provides the maximum power output and the best fuel consumption under non-knocking conditions and the threshold ignition timing (at B in the Figure) which allows only trace knocking under such operating conditions where the knocking tendency is greatest.
However, if the ignition timing is retarded to prevent the engine from knocking, there is the danger of causing difficulties. It is generally known that retardation of the ignition timing results in an increase in the temperature of the exhaust gas from the engine. In particular, if the ignition timing is retarded under the high speed operating conditions of the engine, there is the danger of the rising exhaust gas temperature eventually exceeding the range of allowable temperatures of the system incorporated in the engine.
On the other hand, since any trace knocking does not present any difficult problem to the engine, to retard the ignition timing greatly for the purpose of operating the engine under trace knocking conditions is not desirable in a sense that it results in a decrease in the power output.